Transcript Echo Cancellation - Ian Hung, P.Eng.

echo
cancellation
Ian Hung
2B Computer Engineering
University of Waterloo
August 17th, 2001
Squelching Echo in a Digital Cellular Network
A. Background and Significance
B. Echo Basics
C. Process of Echo Cancellation
Presentation
Overview
D. Echo Control
E. Evaluating Audio Quality
F. Future Trends
This presentation will explore the different methods of handling
echo inherent within a digital cellular network.
A. Background and Significance
Speech quality is the
benchmark at
which the quality of
the network is
assessed.
Wireless phones have gained enormous popularity
and are becoming essential communication tools
that impact our lives from day-to-day personal
tasks to how business is performed.
The key is to maintain customer
loyalty by improving network quality thus
generating additional revenue.
For this reason, a considerable amount of research in the removal of inherent hybrid and acoustic echo with the aim
of improving perceived voice quality has been conducted.
A. Background and Significance
B. Echo Basics
Mobile
Switching
Center
(MSC)
4 Wire
Cell Site
4 Wire
2 Wire
Local
Telephone
Exchange
4 Wire
Long
Distance
Toll Office
Long
Distance
Toll Office
4 Wire
Communications Network General Overview
•Hybrid Echo
•Acoustic Echo
•Other Echo Sources
B. Echo Basics
Long
Distance
Toll Office
4 Wire
Due to the cost of cabling, twowire circuits are used to connect
the telephone to the local
telephone exchange rather than
four-wire trunks.
Hybrid transformers
are used to interface
this two-wire circuit
with a four-wire
station.
4 Wire Send Port
This conversion does not work
perfectly and results in electrical
energy leaking back to the talker
in the form of echo.
2 Wire Port
Hybrid
Device
Echo
4 Wire Receive Port
Hybrid Echo
•Hybrid Echo
•Acoustic Echo
•Other Echo Sources
B. Echo Basics
When there is little round-trip
delay (28ms), it creates a
positive sense that the call is live
by adding sidetone.
Intrusive echo results when the total network delay exceeds 36ms.
The amount of echo depends on how well the hybrid matches both circuits.
This is measured as echo return loss (ERL).
High ERL value = weak signal being reflected.
•Hybrid Echo
•Acoustic Echo
•Other Echo Sources
B. Echo Basics
Audio reflected from window
Audio reflected from dash
Audio reflected from door
This form of echo originates from
handsets and is produced by
poor isolation between the
microphone and speaker.
Acoustic Echo
•Hybrid Echo
•Acoustic Echo
•Other Echo Sources
B. Echo Basics
Multipath echo is transmitted
back to the distant end and
heard as echo.
Background noise is generated when the mobile is operated in hands-free mode.
This noise is passed through the vocoder causing further distortion in speech.
Delay in either speech compression or transmission equipment further induce echo and degrade voice quality.
…especially true when the call is processed through mixed network infrastructures including copper wire, fiber optic lines,
microwave connections, international gateways, and satellite transmission
•Hybrid Echo
•Acoustic Echo
•Other Echo Sources
B. Echo Basics
C. Process of Echo Cancellation
ERLE
Send to
Far End
S-in
Hybrid
Drop-Side
S-out
Line-Side
Echo Canceller
Receive from
Far End
R-out
Endpath
Echo Canceller Implementation in the Digital Wireless Network
C. Process of Echo Cancellation
R-in
Longhaul
Hybrid's impulse
response must be
learned- known as
adaptation
Residual
Echo
S-in
Subtractor
Echo
Estimate
Hybrid
The endpath is
mapped
Echo Canceller – Schematic Diagram
Double-Talk
Detector
S-out
NLP
Error
Signal
Convolution
Processor
H-Register
X-Register
R-out
R-in
Contents within either registers
are multiplied together, a
process called convolution,
producing the echo estimate that
is fed to the Subtractor.
C. Process of Echo Cancellation
Subtractor eliminates the echo
estimate from the convolution
processor
Residual
Echo
S-in
Subtractor
Echo
Estimate
Hybrid
Double-Talk
Detector
R-out
S-out
NLP
Error
Signal
Convolution
Processor
H-Register
X-Register
H-register is adjusted
accordingly, refining the estimate
of the impulse response, in a
process called convergence
Echo Canceller – Schematic Diagram
C. Process of Echo Cancellation
R-in
Double-talk
occurs when
both near and
far-end callers
are speaking at
the same time.
Residual
Echo
S-in
Subtractor
Near-end speech during doubletalk can distort the error signal
and confuse the adaptation
process.
Hybrid
Echo
Estimate
Double-Talk
Detector
NLP
Error
Signal
Convolution
Processor
R-out
Echo Canceller – Schematic Diagram
C. Process of Echo Cancellation
H-Register
X-Register
S-out
Error signal from the Subtractor
is ignored causing the contents
within the H-register to
freeze
R-in
Nonlinear echoes:
clipped speech
signals
Residual
Echo
S-in
speech compression
Subtractor
S-out
NLP
poor quality
speakerphones
Echo
Estimate
pulse code
modulation (PCM)
…challenge
convolution
processor to
develop an accurate
echo estimate.
Hybrid
Double-Talk
Detector
Error
Signal
Convolution
Processor
H-Register
X-Register
R-out
Echo Canceller – Schematic Diagram
NLP: Reduces the residual echo
to inaudible levels based on the
adaptive suppression threshold
R-in
Signals above the threshold are
allowed to pass where as signals
below the threshold are
removed
C. Process of Echo Cancellation
D. Echo Control
Mobile
Switching
Center
(MSC)
Echo
Canceller
4 Wire
Cell Site
4 Wire
Public Switched
Telephone Network
Hybrid
(PSTN)
2 Wire
4 Wire
When a call is made from the mobile user to the
PSTN, the caller will experience echo-free
speech, while the receiver may experience
poor speech quality.
Echo Canceller Implementation in the Digital Wireless Network
•Handling Hybrid Echo
•Handling Acoustic Echo
•Handling Other Complex Echo
D. Echo
EchoControl
Control
Therefore, the echo canceller must work
both ways to accommodate users from
either ends.
During conversation, the audio
picture dynamically changes,
which causes the canceller to
adapt continually.
From MSC
Loudspeaker
Echo
EC should be capable of receiving 270 ms
of delay. Any less and the remaining
echo would be beyond the ability for the
canceller to remove them.
Echo
Canceller
Microphone (Speech w/Echo)
To MSC
(no Echo)
Acoustic echo return loss enhancement
(AERLE), the amount of attenuation
applied to unfavorable echo, has a value
of 65 dB as the minimum requirement
with the non-linear processor enabled.
Echo Canceller Implementation in the Mobile Handset
•Handling Hybrid Echo
•Handling Acoustic Echo
•Handling Other Complex Echo
D. Echo
EchoControl
Control
Performance is dependent upon both network-side EC integration, and mobile-side handset design
Due to strong price pressures,
most handsets do not control echo
very well.
In some cases, such handsets are
known to produce a terminal
compiling loss of 24 dB.
Voice
Echo
Cancelled
Sample
Hybrid
Voice
Sample
2 Wire
Echo
Cancelled
4 Wire
Back-to-Back Implementation in the Digital Wireless Network
•Handling Hybrid Echo
•Handling Acoustic Echo
•Handling Other Complex Echo
D. Echo
EchoControl
Control
E. Evaluating Audio Quality
A. Total Echo Removal
1. No echo at the beginning of the call
2. No Echo throughout the call - call is stable without bursts or distortion
3. Residual Echo (low-level) is absent throughout the call
B. Double-Talk Clarity
1. No echo or scratching noises throughout the call
2. Speech is clearly audible during double-talk
3. No clipping at the beginning and end of speech
*Occasionally, echo cancellers may be fooled due to high background noise. If double-talk detection
is handled incorrectly, near-end speech, perceived to be echo, may be attenuated.
C. Background Audio Transparency
On contrary, background audio transparency is the ability for an echo canceller to allow background
noise to be heard.
Poor DSP = audio fading in and out
Better DSP = consistent background audio.
Noise injection to fill-in for background audio is occasionally used.
Replacing these intelligible background sounds with random noise can be irritating.
D. Echo
E.
Evaluating
Control
Audio Quality
F. Future Trends
In the pursuit for greater call clarity, echo cancellers have been the
center of recent development, and have now become highly
sophisticated transmission equipment at the heart of highly complex
networks.
Effective implementation of echo cancellation across the network holds
the key to improving call quality and ultimately, strengthen customer
satisfaction, boost revenues, and reduce subscriber churn.
D. Future
F.
Echo Control
Trends